Examples:

absolute value  The absolute value of a positive number is the number itself.  For example, the absolute value of 3 is 3.  The absolute value of a negative number is the opposite of the number.  For example, the absolute value of –6 is 6.  The absolute value of 0 is 0.  Vertical bars are used to indicate absolute value.  For example, |3| = 3 and |-3| =3.

abundant number  A number for which the sum of all the proper factors is greater than the number itself.  For example, 12 is an abundant number because the sum of its proper factors is 1 + 2 + 3 + 4 + 6 = 16, and 16 is greater than 12.  See also deficient number, perfect number, and proper factor.

account balance  An amount of money that you have or that you owe.  See also “in the black” and “in the red.”

accuracy (of a count or measurement)  A measure of how close a count or measurement is to the true value; a measure of the correctness of a result.

acre  A unit of area equal to 43,560 square feet.

acute angle  An angle with a measure greater than 0 degrees and less than 90 degrees.

addend  See addition.

addition  A mathematical operation based on “putting things together.”  Numbers being added are called addends; the result of addition is called a sum.  In 12 + 33 = 45, 12 and 33 are addends, and 45 is the sum.  Subtraction “undoes” addition: 12 = 33 = 45, 45-12 = 33, and 45 – 33 = 12.

addition fact  See arithmetic fact.

additive inverses  Two numbers whose sum is 0.  The additive inverse of a number is also called its opposite.  For example, 3 + -3 = 0.  The additive inverse of 3 is –3, and the additive inverse of –3 is 3.

algebraic expression  An expression that contains a variable.  For example, if Maria is 2 inches taller the Joe, and if the variable M represents Maria’s height, then the algebraic expression M – 2 represents Joe’s height.

algorithm  A set of step-by-step instructions for doing something – carrying out a computation, solving a problem, and so on.

altitude  Geography:  height above sea level, also called elevation.  Geometry: height of a distinguishing feature (vertex, opposite side, and so on) from a base.

angle  Two rays with a common endpoint.  The common endpoint is called the vertex of the angle.  An acute angle has a measure greater the 0° and less than 90°.  An obtuse angle has a measure greater than 90° and less that 180°.  A right angle has a measure 90°.  A straight angle has measure 180°.  See also reflex angle.

angle of separation  A measure of how far fingers can be spread apart – shown in the margin is the angle of separation between a person’s thumb and first finger.  

angles, adjacent  Two angles with a common side, that do not otherwise overlap.  In the diagram, angles 1 and 2 are adjacent angels.  So are angles 2 and 3, angles 3 and 4, and angles 4 and 1.

angles, corresponding  Two angles in the same relative position in two figures, or in similar locations in relation to a transversal intersecting two lines.  In the diagram in the margin, Ša and Še, Šb and Šf, Šd and Šh, and Šc and Šg are corresponding angles.  If any two corresponding angles are congruent, then the lines are parallel.

angles, opposite  In a polygon, angles across from each other.

angles, vertical  Two intersecting lines form four adjacent angles.  In the diagram, angles 2 and 4 are vertical angles.  They have no sides in common.  Their measures are equal.  Similarly, angles 1 and 3 are vertical angles.

anthropometry  The study of human body sizes and proportions.

apex  In a pyramid or cone, the vertex opposite the base.

arc  Part of a curve, from one point on a circle to another.  For example, a semicircle is an arc on a circle; its endpoints are the endpoints of a diameter of the circle.

area  A measure of the surface inside a closed boundary.  The formula for the area of a rectangle is A = l * w where A represents the area, l the length, and w the width.  The formula may also be expressed as A = b * h, where b represents the length of the base and h the height of the rectangle.

area model  A model for multiplication problems, in which the length and width of a rectangle represent the factors and the area represents the product.

arithmetic fact  Any of the basic addition and multiplication relationships and the corresponding subtraction and division relationships.  There are –

            100 addition facts, from 0 + 0 = 0 to 9 + 9 – 18

            100 subtraction facts, from 0 – 0 = 0 to 18 – 9 = 9

            100 multiplication facts, from 0 * 0 = 0 to 9 * 9 = 81

            90 division facts, from 0/1 = 0 to 81/9 = 9

Turn-around facts are pairs of related facts such as 3 * 5 = 15 and 5 * 3 = 15.  An extended fact is obtained by multiplying some or all numbers in an arithmetic fact by a power of 10; for example,  20 + 30 = 50, 400 * 6 = 2400, 500 – 300 = 200, 240/60 = 4.

array  See rectangular array.

astronomical unit  The average distance from the Earth to the Sun.  It is used as a unit of measure to express distances in space.

attribute  A feature such as size, shape, color, and so on.

average  See mean.

axis (plural axes)  A number line used in a coordinate grid.